From eb4b2bb7f67a90ea4d9ed36502f11dc470ba81ef Mon Sep 17 00:00:00 2001
From: pca006132 <john.lck40@gmail.com>
Date: Mon, 19 Jul 2021 17:05:48 +0800
Subject: [PATCH] refactored using constrain

to allow easier modification later with subtyping
---
 nac3core/src/typecheck/type_inferencer.rs | 38 +++++++++++++----------
 nac3core/src/typecheck/typedef.rs         |  4 +--
 2 files changed, 22 insertions(+), 20 deletions(-)

diff --git a/nac3core/src/typecheck/type_inferencer.rs b/nac3core/src/typecheck/type_inferencer.rs
index 3ccc3016..6c273ec9 100644
--- a/nac3core/src/typecheck/type_inferencer.rs
+++ b/nac3core/src/typecheck/type_inferencer.rs
@@ -38,6 +38,12 @@ impl<'a> Fold<()> for Inferencer<'a> {
 type InferenceResult = Result<Type, String>;
 
 impl<'a> Inferencer<'a> {
+    /// Constrain a <: b
+    /// Currently implemented as unification
+    fn constrain(&mut self, a: Type, b: Type) -> Result<(), String> {
+        self.unifier.unify(a, b)
+    }
+
     fn build_method_call(
         &mut self,
         method: String,
@@ -55,7 +61,7 @@ impl<'a> Inferencer<'a> {
         let call = self.unifier.add_ty(TypeEnum::TCall { calls: vec![call] });
         let fields = once((method, call)).collect();
         let record = self.unifier.add_ty(TypeEnum::TRecord { fields });
-        self.unifier.unify(obj, record)?;
+        self.constrain(obj, record)?;
         Ok(ret)
     }
 
@@ -114,15 +120,15 @@ impl<'a> Inferencer<'a> {
     fn infer_attribute(&mut self, value: &ast::Expr<Option<Type>>, attr: &str) -> InferenceResult {
         let (attr_ty, _) = self.unifier.get_fresh_var();
         let fields = once((attr.to_string(), attr_ty)).collect();
-        let parent = self.unifier.add_ty(TypeEnum::TRecord { fields });
-        self.unifier.unify(value.custom.unwrap(), parent)?;
+        let record = self.unifier.add_ty(TypeEnum::TRecord { fields });
+        self.constrain(value.custom.unwrap(), record)?;
         Ok(attr_ty)
     }
 
     fn infer_bool_ops(&mut self, values: &[ast::Expr<Option<Type>>]) -> InferenceResult {
         let b = self.primitives.bool;
         for v in values {
-            self.unifier.unify(v.custom.unwrap(), b)?;
+            self.constrain(v.custom.unwrap(), b)?;
         }
         Ok(b)
     }
@@ -181,32 +187,30 @@ impl<'a> Inferencer<'a> {
                     .iter()
                     .flatten()
                 {
-                    self.unifier
-                        .unify(self.primitives.int32, v.custom.unwrap())?;
+                    self.constrain(v.custom.unwrap(), self.primitives.int32)?;
                 }
                 let list = self.unifier.add_ty(TypeEnum::TList { ty });
-                self.unifier.unify(value.custom.unwrap(), list)?;
+                self.constrain(value.custom.unwrap(), list)?;
                 Ok(list)
             }
             ast::ExprKind::Constant {
                 value: ast::Constant::Int(val),
                 ..
             } => {
-                // the index is a constant, so value can be a sequence (either list/tuple)
+                // the index is a constant, so value can be a sequence.
                 let ind: i32 = val
                     .try_into()
                     .map_err(|_| "Index must be int32".to_string())?;
                 let map = once((ind, ty)).collect();
                 let seq = self.unifier.add_ty(TypeEnum::TSeq { map });
-                self.unifier.unify(value.custom.unwrap(), seq)?;
+                self.constrain(value.custom.unwrap(), seq)?;
                 Ok(ty)
             }
             _ => {
                 // the index is not a constant, so value can only be a list
-                self.unifier
-                    .unify(slice.custom.unwrap(), self.primitives.int32)?;
+                self.constrain(slice.custom.unwrap(), self.primitives.int32)?;
                 let list = self.unifier.add_ty(TypeEnum::TList { ty });
-                self.unifier.unify(value.custom.unwrap(), list)?;
+                self.constrain(value.custom.unwrap(), list)?;
                 Ok(ty)
             }
         }
@@ -218,10 +222,10 @@ impl<'a> Inferencer<'a> {
         body: ast::Expr<Option<Type>>,
         orelse: ast::Expr<Option<Type>>,
     ) -> InferenceResult {
-        self.unifier
-            .unify(test.custom.unwrap(), self.primitives.bool)?;
-        self.unifier
-            .unify(body.custom.unwrap(), orelse.custom.unwrap())?;
-        Ok(body.custom.unwrap())
+        self.constrain(test.custom.unwrap(), self.primitives.bool)?;
+        let ty = self.unifier.get_fresh_var().0;
+        self.constrain(body.custom.unwrap(), ty)?;
+        self.constrain(orelse.custom.unwrap(), ty)?;
+        Ok(ty)
     }
 }
diff --git a/nac3core/src/typecheck/typedef.rs b/nac3core/src/typecheck/typedef.rs
index 4c7eaa89..e9abbb16 100644
--- a/nac3core/src/typecheck/typedef.rs
+++ b/nac3core/src/typecheck/typedef.rs
@@ -188,9 +188,7 @@ impl Unifier {
             )
         };
 
-        let (ty_a, ty_b) = {
-            (ty_a_cell.borrow(), ty_b_cell.borrow())
-        };
+        let (ty_a, ty_b) = { (ty_a_cell.borrow(), ty_b_cell.borrow()) };
 
         self.occur_check(a, b)?;
         match (&*ty_a, &*ty_b) {